I've been running a 500GB Western Digital My Book on my desktop, (Dell Precision T3400 running Vista Ultimate), for the last year. I ran windows update yesterday which installed an update for .net 3.5. When the machine rebooted the external drive was not being detected by the OS. I plugged it into another machine and it was detected just fine from there. I did a system restore back to the point prior to the update and that had no affect so I undud the restore. I still don't detect the drive.
For testing purposes I plugged a number of different usb devices into this machine to verify the usb drivers were still functioning appropriately which they appear to be working fine.